Methods for distinguishing between a plurality of materials using CT image or volume data which has been determined using two different X-ray spectra are generally known. However, in some cases, it is difficult to distinguish between specific materials in combinations of other materials using known dual-energy CT methods. For example, this is true of yellow bone marrow (that is to say fat) in the spongiosa with respect to a mixture of iodine in the blood or soft tissue. This distinction is not always possible, even without any image noise, for individual voxels as well as voxel regions over which the absorption is averaged.